1. Field of the Invention (Technical Field)
The invention relates to a pelletizing device comprising a rotary drivable at least partly cylindrical die that delimits a die cavity, the cylindrical part comprising multiple radial openings for the forming of pellets, at least one roller rotatable around a stationary shaft for pressing the material to be pelletized through the radial openings in the die, wherein the roller is attached to the corresponding stationary shaft by means of at least one rolling baring.
2. Description of Related Art
In many industries products are offered in pellet form. These pellets have the form of large grains or small rods and are produced from originally fine grained material. This material in pellet form can easily be handled, be packaged and transported. Examples of pellets can be found in the cattle food industry, the recycling industry, but also plastics industry.
For example in wood industry sawdust and/or wood chips are being pressed to pellets, that then easily can be packaged and transported, but also be fed properly metered to for instance incinerators.
The forming of pellets takes place in pelletizing devices. Common pelletizing devices comprise a cylindrical die that can be rotary driven. Inside the cylindrical die there is a die cavity. This cylindrical die comprises a multitude of radial openings, through which the material to be pelletized is being pressed from the inside to the outside. The pressing often takes place by a roller that is accommodated inside the cylindrical die. The shaft of the roller is parallel to the shaft of the die but the external diameter of the roller is considerably smaller than the internal diameter of the die. In the die cavity the material to be pelletized is now being poured and the die is being driven. The roller is situated at a predefined minimal distance of the inside of the die and between the roller and the die the material to be pelletized is being compressed into the radial openings in the die. The roller is not driven separately but gets into rotation by the rotation of the die and the material to be pelletized that is situated between the die and the roller. With the most common pelletizing devices there are two rollers inside a die parallel to each other and at a distance to each other and each roller is rotatable around the stationary shaft. The material to be pelletized, e.g., sawdust is being mixed with a small amount of water and if this is desirable a binder to form a pasty substance, that then is being formed to pellets under high pressure in the way described before. The rollers are each fixed to their shaft by means of one or more bearings. A usual way is that each roller is connected to the shaft by two adjacent roller bearings, for instance double-row spherical roller bearings or tapered roller bearings.
During the pressing of pellets large forces are being generated. To generate these forces, the die is rotary driven with considerable power. The large pressing forces that occur are being generated by and transmitted to the rollers and these forces are being led via the bearings to the stationary shafts that have been attached to the pelletizing device by a heavy support construction. To have these bearings functioning under this heavy load a good lubrication is mandatory. This lubrication takes place by means of grease.
This lubricating grease is being pumped with a predefined speed through the bearings because of two reasons. Firstly due to the high load it is necessary that there is a regular supply of new grease. However, secondly the atmosphere in the die cavity, because of the fine material to be pelletized that is present there, is such that this fine material must be prevented from entering the bearings, which would lead to premature failing of the bearings. By regularly pumping the grease through the bearings also possible entering dust is being transported away with the grease and it is being prevented that the dust can penetrate to the raceways of the bearings. It is quite usual for a pelletizing machine that every four hours 1 (one) kilogram of grease is being pressed through the bearings. This is not only a considerable cost-increasing factor in the pelletizing process but also the pressing of grease through the bearings contributes to a temperature increase inside the bearings. In addition the grease eventually can mix with the product, which can be undesirable.
The productivity of a pelletizing device is limited amongst others by the maximum permissible temperature in the bearings in the rollers. In order to achieve higher productivity a larger amount of power needs to be fed to the die. This power is to a large extent being transformed into heat, notably in the bearings of the rollers. Because the grease is pumped through these bearings, all spaces inside the bearings are consequently being filled with grease. This means that the rolling elements in the bearings have to be ploughing continuously through the grease, which increases the resistance and consequently also the temperature. So this is an additional limiting factor to the productivity of the pelletizing device.